The present invention relates generally to an acetabular cup and, more particularly, to a press-fit acetabular cup and associated method for securing the cup to an acetabulum.
During the lifetime of a patient, it may be necessary to perform a hip replacement procedure on the patient as a result of, for example, disease or trauma. The hip replacement procedure may involve a total hip replacement or a partial hip replacement. In a total hip replacement procedure, a femoral component having a head portion is utilized to replace the natural head portion of the thighbone or femur. The femoral component typically has an elongated intramedullary stem which is utilized to secure the femoral component to the patient""s femur. In such a total hip replacement procedure, the natural bearing surface of the acetabulum is resurfaced or otherwise replaced with a cup-shaped acetabular component that provides a bearing surface for the head portion of the femoral component.
Acetabular cups may be secured to the acetabulum in a number of different manners. For example, acetabular cups may be secured to the acetabulum by the use of bone cement. However, recent studies have speculated that it may be desirable to secure artificial components to natural bone structures without the use of bone cement. Hence, a number of press fit acetabular cups have been designed for securement to the acetabulum without the use of bone cement.
In either case (i.e. cemented or cementless), the acetabulum is first reamed by the surgeon in order to create a cavity into which the acetabular cup is secured by the use of a surgical tool known as a reamer. It is often difficult for the surgeon to properly match the size of the reamer to the desired acetabular cup size.
Although press fit acetabular cups have heretofore been referred to as being xe2x80x9cgenerally hemisphericalxe2x80x9d in shape, such heretofore-designed cups, in reality, are sub-hemispherical in shape. In particular, as shown in the prior art drawing of FIG. 7, a heretofore designed acetabular cup 100 has an apex or dome 102 at a proximal end 104 thereof along with an annular rim 106 at a distal end 108 thereof. In between the dome 102 and the annular rim 106, the prior art acetabular cup 100 has a sidewall that has a convex proximal surface and a concave distal surface.
However, as shown in FIG. 7, the configuration of the prior art acetabular cup 100 is sub-hemispherical. In particular, a xe2x80x9ctruexe2x80x9d hemisphere 114 is shown in FIG. 7 as a phantom line overlay. As can be seen, a distal face 116 of the annular rim 106 does not, in fact, lie along the 180xc2x0 surface (or loosely, the equator 118) of the hemisphere 114, but rather is recessed away from the equator 118 by a relatively significant distance X. In fact, it is not uncommon for prior art cup designs to be recessed from the equator 118 of the cup by as much as 4-5 millimeters (i.e. X=4-5 mm).
Such a configuration has a number of drawbacks associated therewith. For example, such a large recess distance X (i.e. 4-5 mm) renders it difficult for the surgeon to ream a properly sized cavity in the acetabulum. In particular, the cutting head of heretofore-designed reamers are typically configured as relatively true hemispheres. Hence, when a surgeon reams the patient""s acetabulum, the surgeon has to xe2x80x9cestimatexe2x80x9d the approximate depth of the reamed recess. More specifically, if the surgeon reams all the way to the 180xc2x0 surface or xe2x80x9cequatorxe2x80x9d of the reamer, the annular rim 106 of the acetabular cup 100 will be recessed in the reamed cavity. Conversely, if the surgeon does not ream deeply enough (i.e. xe2x80x9cunder reamsxe2x80x9d), the acetabular cup 100 will not be fully seated in the reamed cavity of the acetabulum. In light of the fact that surgeons occasionally select a reamer that is slightly smaller in size than the acetabular cup to be implanted, under reaming may also disadvantageously lead to bone fracture of the acetabulum since excessive force is often utilized to insert the cup into the undersized (i.e. under reamed) cavity. Some of the early bone cemented cups did not suffer from this problem by being configured more closely as xe2x80x9ctruexe2x80x9d hemispheres. However, as indicated above, such cups undesirably required the use of bone cement during implantation thereof.
Another drawback associated with heretofore-designed press fit acetabular cups relates to the configuration of the outer shell. In particular, in an attempt to increase retaining forces, a number of acetabular cups have been designed with a flared rim (known as dual radius or xe2x80x9cbubblexe2x80x9d cups) or a frusto-conically shaped annular rim portion (known as dual-geometry cups). Although the configuration of such cups may generate relatively strong retention forces at the rim portion of the cup, surface contact and therefore retention forces are relatively small at the portions of the outer shell other than the rim portion, particularly in the dome area. Moreover, such reduced surface contact at the portions of the outer shell other than the rim portion reduces bone ingrowth in such portions.
With the above-mentioned heretofore-designed press-fit acetabular cups, a two-part reaming process is typically necessary. The two-part reaming process involves reaming of the acetabulum using a reamer of a first size, then reaming the acetabulum using a reamer of a second size. The more reaming, the more likely that a problem will occur. For example, many conventional cementless acetabular cup systems use a cup that is two millimeters (2 mm) larger than the last reamer size used. Inserting this size cup into the undersized reamed acetabulum to accommodate this system is sometimes difficult, particularly with resistance in the dome area of the cup, which is also larger than the last reamer size used.
What is needed therefore is an acetabular cup and associated method that overcomes one or more of the above-mentioned drawbacks.
What is further needed is an acetabular cup and associated implant method that allows for the cup to be secured to the acetabulum without the use of bone cement.
What is also needed is an acetabular cup and associated implant method that facilitates greater amounts of bone ingrowth relative to heretofore designed acetabular cups.
What is still further needed is a cementless acetabular cup that utilizes a single reaming process for implant.
The present invention is a press-fit acetabular cup and method of securing the acetabular cup to an acetabulum so as to provide a bearing surface for a head portion of a femur. The acetabular cup is shaped to provide a cementless, press-fit into a reamed acetabulum. The acetabular cup is formed of a body having a sidewall defining a radius from a center point of the annular rim to the sidewall wherein the radius increases in length from the apex to said annular rim. The method of securing the acetabular cup first includes reaming the acetabulum with a reamer having a head with a radius of curvature that is less than the radius from the center point to the annular rim of the acetabular cup.
According to one embodiment of the present invention, there is provided an acetabular cup. The acetabular cup includes a body defining a dome having an apex and an annular rim. The dome is defined by an increasing radius sidewall that extends from the apex to the annular rim. The annular rim defines a plane having a center point. Wherein a radius from the center point to the annular rim has a first given length, 1L , the center point to the apex defining a second given length, 2L, and the second given length is less than the first given length.
According to another embodiment of the present invention there is provided an acetabular cup. The acetabular cup includes a dome-shaped shell having an apex and an annular rim. The dome-shaped shell has a sidewall defining a radius from a center point of the annular rim to the sidewall, wherein the radius increases in length from the apex to the annular rim.
According to yet another embodiment of the present invention, there is provided a method of securing an acetabular cup to an acetabulum. The method includes reaming a cavity of a first radius of curvature into an acetabulum with a reamer, the reamer including a reamer head having the first radius of curvature, and press fitting an acetabular cup into the reamed cavity, the acetabular cup having a body defining a dome having an apex and an annular rim, the dome defined by a gradually increasing radius sidewall that extends from the apex to the annular rim, the annular rim defining a plane having a center point, wherein a radius from the center point to the annular rim has a first given length, 1L, the center point to the apex defining a second given length, 2L, the second given length is less than the first given length, the first given length is greater than the first radius of curvature, and the second given length is approximately equal to the first radius of curvature.
According to a further embodiment of the present invention, there is provided a method of securing an acetabular cup to an acetabulum. The method includes reaming a cavity of a first radius of curvature into an acetabulum with a reamer, the reamer including a reamer head having the first radius of curvature, and press fitting an acetabular cup into the reamed cavity, the acetabular cup having a dome-shaped shell having an apex and an annular rim, the dome-shaped shell having a sidewall defining a shell radius from a center point of the annular rim to the sidewall, wherein the shell radius gradually increases in length from the apex to the annular rim, and the shell radius at the annular rim is greater than the radius of curvature.
The present cementless acetabular cup provides an enhanced peripheral press-fit with reduced loading in the dome area. This helps provide initial stability for bone ingrowth and long-term fixation. Further, a good peripheral fit may help protect against the migration of wear debris from a xe2x80x9cpumping actionxe2x80x9d of the effective joint space.
Additionally, reamer to cup dimensions of the present acetabular cup have a line-to-line fit in the dome area with a gradual increase in press-fit or size from the dome to the rim. This allows the present acetabular cup to achieve maximization of peripheral contact for stability while minimizing dome loading and helping reduce the risk of acetabular fracture.
The present acetabular cup also accepts a polyethylene liner or insert such that the cup and liner achieve congruency therebetween without rim loading the liner. A positive locking mechanism and anti-rotation devices in the metal shell/body defining the acetabular cup secure the liner to the shell. Congruency and secure locking of the polyethylene liner work together to reduce micromotion at the shell/liner interface. Further, such high conformance between the shell and the liner results in efficient load transfer and reduce contact stresses.